Concentrated sodium (Na), a component of salt, can damage plant tissue whether it contacts above or below ground parts. High salinity can reduce plant growth and may even cause plant death. Care should be taken to avoid excessive salt accumulation from any source on tree and shrub roots, leaves or stems. Sites with saline (salty) soils, and those that are exposed to coastal salt spray or paving de-icing materials, present challenges to landscapers and homeowners.
Saline soils occur when salts accumulate in the soil. Significant salt accumulation is uncommon in areas where rainfall exceeds 20 inches per year. Since Virginia averages 40-50 inches of rain most years, saline soils generally are not a widespread problem. However, saline soils do occur in specific situations such as:
How do saline soils affect trees and shrubs?
Plant root cells contain a membrane which allows water to pass through, but which prevents salt from entering. As the soil's salt content increases, it becomes more difficult for water to pass through the membrane into the root. In addition, if salt levels get high enough they may actually dehydrate roots or cause "salt burn" by drawing water out of root cells.
High levels of soluble salts also cause changes to soil structure, resulting in compacted soils that are problematic for plants. Because salts bind with soil clays, causing them to swell, compaction occurs more frequently in clayey soils than in sandy soils. Compaction causes reduction of pore spaces between soil particles, reducing water and oxygen penetration into the soil, and water drainage from the soil. As a result, water and oxygen availability to plant roots, and consequently plant growth and pest resistance, is affected.
Plants vary in their ability to grow in salty soils. Plants that grow only in saline soils are called "halophytic" or salt loving. Halophytic plants are generally found in coastal areas, in salt-water marshes, and in brackish (moderately saline) wetlands. The presence of some of these plants (such as spartina and sea oats) is generally indicative of a saline soil.
Most landscape plants are sensitive to soil salinity. Seedling trees and shrubs and young transplants can be particularly sensitive to salt exposure. The severity of salt damage to plants depends upon the amount and duration of exposure, and the concentration of salt. For example, coastal areas that receive consistent salt spray may always have elevated levels of soil salinity, whereas areas adjacent to roads where de-icing salts are applied may incur salt exposure only sporadically during winter storms. Similarly, areas subject to flooding by brackish water may only be affected by salinity following storms and high tides.
If there is adequate precipitation to leach the salt out of these areas soon after the initial exposure, the amount and duration of salt exposure will be brief. If salt exposure persists, or is repeated, damage will be more severe. There is a direct relationship between the amount and duration of salt exposure and potential damage to plants. The higher the amount of salt in the soil, the greater the impact on plants. Salt damage is generally more severe during periods of hot, dry weather.
Measuring soil salinity
The amount of salt in the soil can be measured with a soil test. The Virginia Cooperative Extension Service Soil Test Laboratory reports salt levels using the measure "parts per million" or "ppm." Salt concentrations of 1-1000 ppm are considered low, and those from 1000-2000 ppm medium. With the exception of very salt sensitive plants, most landscape plants can tolerate salt concentrations in the medium range.
Symptoms of saline soil damage
Plant damage due to saline soils becomes evident more slowly than plant damage due to salt spray. At elevated levels, soil salts are harmful to seed germination and plant growth. General symptoms include stunted growth and reduced yields. All parts of the plant, including leaves, stems, roots and fruits, may be reduced in size. The signs and symptoms displayed by deciduous and broad-leaved trees and shrubs include leaf necrosis (death), marginal leaf or needle burn, leaf drop, and eventual plant death. Entire leaves can be affected and drop prematurely. Buds may fail to open or grow, and branches may die. Sometimes deciduous trees may exhibit early fall color and leaf drop. Salt damage on deciduous trees and shrubs usually becomes evident in late summer following the growing season, or during periods of hot, dry weather (summer drought).
On conifers (firs, junipers, pines, spruces), damage appears as brown needle tips. The brown discoloration progresses toward the base of the needles as salt exposure increases. Salt damage on evergreen trees and shrubs [both conifers and broadleaf (hollies, photinia, southern magnolia)] usually first appears in late winter to early spring and becomes more extensive during the growing season. In extreme situations, trees and shrubs will die due to soil salt damage.
When trying to diagnose plant damage, keep in mind that all of the above signs and symptoms can also be caused by a variety of other factors including root damage, drought, diseases, chemical misuse, etc. Try to eliminate these other possibilities, and use tools such as soil and water analyses, and weather data to help you arrive at a correct damage diagnosis.
Reducing soil salinity or soil salt damage
Numerous options exist for reducing salt damage including:
The aerial drift of salt-laden water droplets that are deposited on trees and shrubs causes salt spray damage. When droplets evaporate, the salt's sodium and chlorine ions can penetrate stems, buds and leaves, causing direct damage. Salt spray damage to trees and shrubs is most frequently seen on seaside plants and near sidewalks and roads where de-icing salts are applied. Additional stresses in these areas, including wind, sun, heat, exposure, heavy traffic and saline soils, increase the likelihood of damage.
How does salt spray affect trees and shrubs?
Exposure to salt spray can cause stem and foliage disfigurement, reduced growth, and often plant death. Because aerial salt spray damage may appear similar to damage caused by other stresses, a tree or shrub's location and damage symptoms should be carefully evaluated to correctly identify the damage's cause. Consider the distance from sidewalks, roads, and parking lots, or salty water sources, the frequency and severity of storms and winds that carry aerial salt drift inland, the traffic levels and speeds on adjacent roads, and how often winter de-icing salts are used. Remember that salts used for de-icing can cause damage when salty ice or snow contacts adjacent vegetation.
Symptoms of salt spray damage
Examine injury patterns on trees and shrubs. Winter salt spray damage to deciduous plants causes bud death and twig dieback. Tree and shrub growth after this damage will have a "witches-broom" (tufted) appearance. On foliage, salt spray causes leaf burn or scorch, or needle browning. Direct signs such as white salt residue are a strong indication that salt spray may be injuring landscape plants.
For roadside areas, salt spray damage is often localized on the side of the plant facing the road, and on portions of the plant within the spray drift line. Trees located farther from roads will display fewer symptoms. Symptoms become more pronounced when more salt is applied. Road de-icing salt spray damage is usually seen in late winter on evergreens and during spring growth on deciduous trees. For seashore areas, salt spray damage is seen soon after storms, and occurs inland if salt spray is carried farther by strong winds.
Reducing salt spray or salt spray damage
Numerous options exist for reducing salt damage including:
Trees tolerant of saline soils or salt spray
|Common name||Latin name||Deciduous/ Evergreen||Type of salt tolerance||Cold hardiness/Heat tolerance|
|Hedge maple||Acer campestre||D||Salt spray||5-8/8-4|
|Sycamore maple||Acer pseudoplatanus||D||Salt spray||4-7/7-1|
|Horsechestnut||Aesculus hippocastanum||D||Salt spray||3-8/8-1|
|Red buckeye||Aesculus pavia||D||Saline soils||5-8/8-4|
|Paper birch||Betula papyrifera||D||Salt spray||2-7/7-1|
|Gray birch||Betula populifolia||D||Salt spray||3-7/7-2|
|Catalpa||Catalpa speciosa||D||Salt spray||4-8/8-1|
|Hackberry||Celtis laevigata||D||Salt spray||5-9/9-3|
|White fringetree||Chionanthus virginicus||D||Saline soils||5-9/9-5|
|Lavalle hawthorne||Crataegus x lavallei||D||Salt spray||5-8/8-3|
|Japanese cedar||Cryptomeria japonica||E||Salt spray||6-9/9-6|
|Common persimmon||Diospyros virginiana||D||Saline soils, salt spray||7-9/9-7|
|White ash||Fraxinus americana||D||Saline soils, salt spray||6-9/9-3|
|European ash||Fraxinus excelsior||D||Salt spray||6-9/9-6|
|Green ash||Fraxinus pennsylvanica||D||Salt spray||4-9/9-1|
|Ginkgo||Ginkgo biloba||D||Salt spray||5-9/9-2|
|Honeylocust||Gleditsia triacanthos||D||Saline soils, salt spray||3-7/7-1|
|Kentucky coffeetree||Gymnocladus dioicus||D||Salt spray||5-9/9-2|
|American holly||Ilex opaca||E||Salt spray||5-9/9-5|
|Black walnut||Juglans nigra||D||Saline soils, salt spray||5-9/9-5|
|Eastern red cedar||Juniperus virginiana||E||Saline soils, salt spray||3-9/9-1|
|Goldenraintree||Koelreuteria paniculata||D||Saline soils, salt spray||5-9/8-5|
|Common larch||Larix decidua||D||Salt spray||3-6/6-1|
|Sweetgum||Liquidambar styraciflua||D||Salt spray||6-9/9-1|
|Southern magnolia1||Magnolia grandiflora||E||Saline soils, salt spray||7-9/9-3|
|Sweetbay magnolia||Magnolia virginiana||E||Saline soils||6-9/9-6|
|Black gum||Nyssa sylvatica||D||Salt spray||5-9/9-5|
|Colorado spruce2||Picea pungens||E||Salt spray||3-8/8-1|
|Austrian pine||Pinus nigra||E||Salt spray||5-8/8-4|
|Longleaf pine1||Pinus palustris||E||Salt spray||7-9/9-3|
|Japanese black pine||Pinus thunbergiana||E||Saline soils, salt spray||5-8/8-4|
|White poplar||Populus alba||D||Saline soils, salt spray||4-9/9-1|
|Carolina cherrylaurel1||Prunus caroliniana||D||Saline soils||7-9/9-3|
|Black cherry||Prunus serotina||D||Salt spray||3-8/8-2|
|White oak||Quercus alba||D||Saline soils||5-9/9-5|
|Bur oak||Quercus macrocarpa||D||Saline soils, salt spray||3-9/9-1|
|Pin oak||Quercus palustris||D||Saline soils||5-8/8-4|
|Willow oak||Quercus phellos||D||Salt spray||6-9/9-5|
|English oak||Quercus robur||D||Salt spray||4-8/8-4|
|Red oak2||Quercus rubra||D||Saline soils||5-9/9-4|
|Live oak1||Quercus virginiana||E||Saline soils, salt spray||7-9/9-3|
|Black locust||Robinia pseudoacacia||D||Saline soils, salt spray||4-9/9-4|
|Weeping willow||Salix alba||D||Salt spray||6-9/9-5|
|Corkscrew willow||Salix matsudana||D||Salt spray||6-9/9-5|
|Japanese pagodatree||Sophora japonica||D||Salt spray||6-9/9-6|
|Japanese tree lilac||Syringa reticulata||D||Saline soil, salt spray||6-8/8-6|
|Baldcypress||Taxodium distichum||D||Saline soils, salt spray||5-11/12-5|
|Chastetree1||Vitex angus-castus||D||Saline soils||6-9/9-1|
Shrubs tolerant of saline soils or salt spray
|Common name||Latin name||Deciduous/ Evergreen||Cold/Hardiness/ Heat tolerance|
|Red chokeberry||Aronia arbutifolia||D||5-9/9-4|
|Littleleaf boxwood||Buxus microphylla||E||6-9/9-5|
|False cypress||Chamaecyparis pisifera||E||4-8/8-1|
|Red osier dogwood||Cornus sericea||D||5-8/8-3|
|Spreading cotoneaster||Cotoneaster divaricatus||D||6-8/8-3|
|Rockspray cotoneaster||Cotoneaster horizontalis||D||5-7/7-5|
|Scotch broom||Cytisus scoparius||D||6-8/8-6|
|House hydrangea||Hydrangea macrophylla||D||6-9/9-3|
|St. John's wort||Hypericum calycinum||D||5-9/9-4|
|Chinese holly1||Ilex cornuta||E||7-9/9-7|
|Japanese holly||Ilex crenata||E||5-7/7-5|
|Yaupon holly1||Ilex vomitoria||E||7-10/12-7|
|Chinese juniper||Juniperus chinensis||E||3-7/7-1|
|Common juniper||Juniperus communis||E||3-9/9-1|
|Shore juniper||Juniperus conferta||E||5-9/9-3|
|Creeping juniper||Juniperus horizontalis||E||3-9/9-1|
|Amur privet||Ligustrum amurense||D||3-7/7-2|
|Tatarian honeysuckle||Lonicera tatarica||D||3-9/9-1|
|Wax myrtle1||Myrica cerifera||E||6-9/9-6|
|Mock orange||Philadelphus coronarius||D||5-8/8-3|
|Mugo pine||Pinus mugo||E||3-7/7-1|
|Shrubby cinquefoil||Potentilla fruticosa||D||3-7/7-1|
|Purple-leaf sand cherry||Prunus x cistena||D||4-8/8-1|
|Cherry laurel||Prunus laurocerasus||E||6-9/9-5|
|Beach plum||Prunus maritima||D||3-6/6-1|
|Indian hawthorn1||Rhapiolepis indica||E||7-11/12-7|
|Staghorn sumac||Rhus typhina||D||3-8/8-1|
|Lady Banks rose1||Rosa banksiae||D||7-9/9-3|
|Rugosa rose||Rosa rugosa||D||3-9/9-1|
|Scotch rose||Rosa spinosissima||D||3-9/9-1|
|Japanese spirea||Spiraea japonica||D||3-8/8-1|
|Bumalda Japanese spirea||Spiraea x bumalda||D||3-8/8-1|
|English yew2||Taxus baccata||E||5-7/7-5|
|Japanese yew2||Taxus cuspidata||E||4-7/7-5|
|Highbush blueberry||Vaccinum corymbosum||D||5-9/9-2|
|European cranberry bush viburnum||Viburnum opulus||D||4-8/8-1|
1May not be suitable for northern or western Virginia (check your cold hardiness zone).
2May not be suitable for southeastern Virginia (check your heat tolerance zone).
Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009